1. Field of the Invention
The present invention relates to cardiac pacemakers and, more particularly, to cardiac pacemakers with the facility to continuously detect a measure of battery expenditure so as to predict nominal pacemaker end-of-life (EOL).
2. Description of the Prior Art
In the area of implantable cardiac pacemakers, it has long been recognized that it is important to have the capacity to determine a measure of the effective remaining lifetime of the implanted pacemaker. Although great improvements have been made in the batteries that are used in pacemakers, extending significantly the available lifetime of pacemakers, there remains a need to have accurate information from which to predict EOL. As used herein, and as is common in the pacemaker art, end-of-life refers to a time when the battery has been drained sufficiently that the pacemaker should be replaced. Thus, EOL generally does not refer to a time when a pacemaker actually ceases to operate effectively, but a time by which the pacemaker should be replaced while there is still a sufficient factor of safety. Thus, e.g., the pacemaker manufacturer may recommend a replacement of the pacemaker at a time when it actually has an estimated three months of lifetime remaining, to guarantee that replacement takes place before there is a significant danger of actual cessation of pacemaker operation.
An early technique for EOL detection in pacemakers was founded upon the characteristics of the battery impedance. Based upon the observation that battery internal impedance was substantially constant throughout most of the battery lifetime, and then began to increase as battery exhaustion approached, the standard technique utilized was to isolate the battery from the pacemaker briefly in order to get an internal impedance measurement. See, for example, U.S. Pat. No. 5,137,020, assigned to Medtronic, Inc. However, in the latest generation of batteries utilized in implantable pacemakers, the impedance change occurs too quickly to provide sufficient warning, and cannot be reliably detected safely in advance of effective battery exhaustion. Consequently, another engineering approach to the matter is required.
The more recent solution to the problem of measuring EOL in the pacemaker art is based on the fact that the battery energy at start-of-life is accurately known. The pacemaker is provided with a circuit for obtaining a measure of total, or integrated battery current expenditure, and determining from this measure what percentage of battery energy has been depleted. By making it possible for a physician to determine what percentage of the battery capacity has been expended, the physician can project a remaining safe lifetime. The basic concept of detecting EOL by accumulating a measure of energy usage in an implanted pacemaker is illustrated in U.S. Pat. Nos. 4,556,061 and 4,715,381. In U.S. Pat. No. 4,556,061, there is provided a battery consumption monitor circuit which develops with an internal counter a cumulative count representative of energy consumption by the pacemaker. A sense resistor is used to develop a voltage representative of pacemaker current flow, which voltage is imposed upon a voltage-controlled oscillator (VCO), the output of which is inputted into the internal counter. The counter continuously accumulates the pulses so as to provide a measure of the integral of battery current flow, and thus total energy expenditure. However, the accuracy of the VCO is a function of its capacitor, such that the use of VCO in this manner makes the EOL detector capacitor-dependent. It is very difficult and expensive to provide an extremely precise capacitor value for an implantable pacemaker application. Further, the expected variability of the capacitor value over the lifetime of an implanted pacemaker, which may be up to ten or more years, leads to a loss of accuracy. Accordingly, what is needed is a more reliable form of detecting battery consumption and, in particular, an EOL detector which is not capacitor-dependent. Optimally, what is desired is an EOL circuit where, to the extent any capacitor is utilized, the capacitor can change in value over the life of the pacer and not affect the accuracy of the measurement, i.e., the EOL detector is capacitor-independent and otherwise highly reliable.
U.S. Pat. No. 4,715,381 illustrates a technique of making calculations of approximate battery energy expenditure, rather than actually measuring battery consumption. This reference shows a stimulation pulse counter which counts the number of delivered stimulus pulses. This information is utilized together with the programming parameters to determine the total amount of energy of the delivered pulses over an elapsed time. This calculated signal is added to a fundamental consumption signal which is based upon certain approximations and assumptions, and used to derive a signal representative of approximate total battery expenditure. This technique clearly provides at best an approximation, and is inherently subject to a greater probability of inaccuracy than the energy consumption technique. Further, neither of the prior art techniques are as energy efficient as desired, i.e., the EOL detector itself involves counters, oscillators and the like which are constantly operating and consume an undesirable amount of energy. Thus, there remains a substantial need in the pacemaker art for an improved EOL detector which is more reliable and accurate, can be built with minimum cost, and which itself draws an optimally minimum amount of energy from the pacemaker battery.